This invention relates to a method of and apparatus for controlling the thermal gradient which develops when, for example, a vent gas is introduced into a load lock chamber containing a heated object.
Consider, for example, a work station in a wafer processing system for the production of memory devices. Such a work station may typically be provided with a load lock chamber, and silicon wafers which are brought one by one out of a chemical vapor deposition chamber are passed through this load lock chamber before they are transferred out, say, by a storage elevator. The wafer removed from a chemical vapor deposition chamber may be about 360.degree. C., and the wafers do not cool appreciably on the storage elevator if it is kept in a vacuum condition. Since the wafers are brittle and easily damaged, they are supported only by peripheral surface areas. In vacuum, conduction of heat to the storage elevator relies on intimate contact between the wafer and the elevator. If the wafer surface contour does not exactly match the planar surface contour of the storage elevator, the contact between the surfaces is poor and hence the conductivity is low. Even if the contours match, surface roughness and lightness of the wafer minimize its contact with the elevator surface.
If the load lock chamber is vented, the wafers suddenly begin to cool. The vent gas conducts heat from the wafer to the storage elevator, cooling off the parts of the wafer that are in contact with the storage elevator, while the other part of the wafer loses heat only by transverse conduction through the silicon. This results in a large thermal gradient in the wafer. This is believed to be the cause of large numbers of wafers broken in the storage elevator. The parts of the wafer in contact with the storage elevator contract as they cool, while the other non-contacting part does not. This differential cooling results in a tensile load in the cool part and a corresponding compressive load in the hot part. These loads in combination can either buckle or shear the wafer, depending on the orientation of the gradient.